The Result typeclass, for converting a single value in a row
returned by a SQL query into a more useful Haskell representation.

A Haskell numeric type is considered to be compatible with all
PostgreSQL numeric types that are less accurate than it. For instance,
the Haskell Double type is compatible with the PostgreSQL's 32-bit
Int type because it can represent a Int exactly. On the other hand,
since a Double might lose precision if representing a 64-bit BigInt,
the two are not considered compatible.